1. Field of the Invention
The invention relates generally to directional drilling. More particularly, the invention relates to back reamers used in horizontal directional drilling. More particularly still, the invention relates to a modular back reamer capable of being configured to a variety of drilling diameters for use in horizontal directional drilling.
2. Background Art
Horizontal directional drilling (“HDD”) is a process through which a subterranean bore is directionally drilled in a substantially horizontal trajectory from one surface location to another. Typically, HDD operations are used by the utilities industry to create subterranean utility conduits underneath pre-existing structures, but any application requiring a substantially horizontal borehole may utilize HDD. Frequently, HDD bores are drilled to traverse rivers, roadways, buildings, or any other structures where a “cut and cover” methodology is cost prohibitive or otherwise inappropriate.
During a typical HDD operation, a horizontal drilling rig drives a drill bit into the earth at the end of a series of threadably connected pipes called a drill string. As the operation is substantially horizontal, the drilling rig supplies rotational (torque on bit) and axial (weight on bit) forces to the drill bit through the drill string. As the drill bit proceeds through the formation, additional lengths of drill pipe are added to increase the length of the drill string. As the drill string increases in flexibility over longer lengths, the drill string can be biased in a predetermined direction to direct the path of the attached drill bit. Thus, the drilling is “directional” in that the path of the bit at the end of the drill string can be modified to follow a particular trajectory or to avoid subterranean obstacles.
Typically, HDD operations begin with the drilling of a small “pilot” hole from the first surface location using techniques described above. Because of the diminished size in relation to the final desired diameter of the borehole, it is much easier to directionally drill a pilot bore than a full-gage hole. Furthermore, the reduced size of the pilot bit allows for easier changes in trajectory than would be possible using a full-gage bit. At the end of the pilot bore, the drill string emerges from the second surface location, where the pilot bit is removed and a back reamer assembly is installed. Usually, the back reamer assembly is a stabilized hole opener that is rotated as it is axially pulled back through the pilot bore from the second surface location to the first surface location. The drilling rig that supplied rotary and axial thrusting forces to the pilot bit during the drilling of the pilot bore supplies rotary and axial tensile forces to the back reamer through the drill string during the back reaming. Preferably, the stabilizer of the back reamer is designed to be a close fit with the pilot bore so the back reamer follows as close to the pilot bore trajectory as possible.
Formerly, back reamers were large, custom-built assemblies that were fabricated, assembled, and welded together to suit a particular job and subsequently discarded when the job was finished or the reamer was damaged. Because each job was substantially unique, there was little benefit in retaining the reamers after the job was completed. Furthermore, because each job-specific back reamer was only configured to drill one hole size, custom, one-shot fabrication was preferred over maintaining a large inventory of varied sizes and configurations.
Over time, numerous attempts to create re-configurable back reamers have been made. As a result, various concepts for back reamers having replaceable components (e.g. cutting arms, cones, and stabilizers) have been introduced to the market but with mixed results. Particularly, HDD back reamers with replaceable cutters affixed to the reamer body through heavy welds. While the cutters are replaceable in theory, the welds must be broken and removed before replacement cutters can be installed. Other HDD back reamers are constructed as standard oilfield hole openers in that saddle-mounted cutters are employed. While the cutters are replaceable, there is no flexibility to change the type of cutters (e.g. rotating or drag) or the cutting diameter.